Since Cythera is out, I thought I'd share a little piece that involves both the cultures/worldbuilding and also some fun chemistry between the characters.

In which Cythera is briefing Hereu on what's expected of a treaty groom, with the able assistance of Athain, the ship's captain...

Cythera took a deep breath. "Let's begin with an overview of Agni itself, as the environmental factors are key to understanding the kinship structure. Agni is a very cold world and very volcanically active. Other than some extremely hardy plants all agriculture is subterranean in climate controlled circumstances. Originally, the inhabitants used the caverns warmed by hot springs and volcanic activity to both live in and to grow crops. More recently the Agni have developed very sophisticated hydroponics and have created entire subterranean ecosystems within the caverns. All beneath an extremely cold surface!"

"Not to mention very advanced technology in other regards," Athain said. "Their starships are easily the equal of ours."

"Because these volcanic oases could only support a limited population at each one, the Agni developed a complex matrilineal society with complicated rules for how males would move between groups in order to prevent inbreeding. The treaty groom is an old and honored fixture of Agni culture -- a young man of one group married in essence to all the women of another group in order to ensure genetic diversity. This does not, in actual practice, mean that he is expected to mate with all the women of that group."

Hereu blinked. "I thought…."

"That they would all fall upon you at once?" Athain looked like he was trying not to smile. "A pleasant fantasy, but impractical."

"The Agni Empress herself is more than eighty years old," Cythera said. "While it is she that you will go through the formal rite of sealing with, because she is the eldest of her house, it is unlikely this mating will be other than symbolic." Cythera took a deep breath, trying to remember all the details of her briefing files. She would have to check many proper names later, but for now an overview would suffice. It seemed like this was more than Hereu had already heard. And whose job had it been to tell him? Orlion or the Merrow or someone else? Had he not been told or had he refused to listen? In any event, at least he was listening now.

"The Empress has three daughters. It is our best guess that all of them are past childbearing age. Therefore it seems likely that the sealing will be consummated with one or more of the granddaughters, of which there are eight."

Hereu's eyes grew very wide.

"Not at once," Athain said. The corner of his mouth twitched. "There's not a man alive that could do that."

"It is expected that a treaty groom will rotate among the households of his new group -- in this case living for some period of time in the House of each of the daughters. While he is there he is expected to do his best to get the selected representatives from that House with child." Cythera took a deep breath. "It is of course worth remembering that most of these women already have attachments. They may have consorts of long standing, and some of them are old enough to be your mother. They have no great desire to mate with you either. It's a marriage of state."

"So why would they?" Hereu looked spooked, but at least he was talking sensibly.

"As near as we can understand of Agni culture, the daughter born of a treaty marriage is considered special, the most likely heir -- probably originally because it meant that the exogamous marriage would be guaranteed to produce a child with fresh genes and none of the defects that come with inbreeding, but that's neither here nor there," Cythera said. "The Agni Empress can name any of her three daughters as her heir. The one with a granddaughter or great granddaughter who is your child is most likely to be chosen. At least that is our understanding," she amended. "There is a great deal about Agni culture we don't know. But you, as the father of the child who will someday be Empress, could have enormous influence."

"I'd guess each House will have picked one or two women," Athain said.

"And they're all going to be old?" Hereu's eyes were wide. "Thirty-five or forty?"

Athain cleared his throat as though he were trying not to laugh. "There is life after forty," he said. "I'm forty-four myself. And while I'd never presume to guess the age of an Adept, if Cythera was already a full Adept seven years ago, she must be well into her thirties."

"I am thirty-five," Cythera said primly. "And I assure you that I'm still quite capable." She couldn’t resist a sideways glance at Athain as she spoke.

"I'm sure you are, Adept," he replied. His eyes flickered to hers for a moment, a teasing light that belied his very formal words. "You don't appear to have declined appreciably."

"Women decline more slowly," Cythera said with a sweet smile. "Unfortunately virility doesn't endure as well."

"A sad fact of nature. But one tries. Experience may offer much in terms of compensation," Athain replied with a half bow.

"And with experience comes expertise," Cythera said. She couldn’t help the expression of delight that changed her face. "There is much to be said for expertise, captain."

"As captain, it's my duty to place all my expertise at your disposal," Athain said very seriously. But his eyes were laughing.

Orlion cleared his throat.

Hereu was looking back and forth from one of them to the other perplexedly. "What?"

"The Adept was briefing you on the Agni royal family," Orlion prompted. "Cythera?"